1. Field of Invention
This is a test apparatus and method for a laser that helps determine spatial coherence, temporal coherence, and modeling of noisy environments.
2. Description of the Prior Art
Most applications of lasers rely on the laser light beam having good spatial and temporal coherence. Spatial coherence means that the laser beam relative amplitude and phase at different lateral (to the beam propagation direction) positions stays constant over time. Temporal coherence means that the beam amplitude and phase at a lateral position doesn't vary with time. The optical field generated by a coherent beam has a simple structure such as a plane or spherical wavefront and a deterministic amplitude and phase profile with minimal time variation. Temporal coherence is measured in an interferometer, which interferes a light beam with a copy of itself delayed in time. Measurement of the time averaged intensity of the interfered light beam gives the temporal coherence. Spatial coherence is measured by interfering two portions of a light beam at different lateral positions. This can be measured by cross correlation of portions of the light beam separated by a distance. There is a need for a simple tool that can diagnose temporal and spatial incoherence of a laser's output.
Lasers operating outside laboratories are often in environments that cause scattering of the originally coherent beam. This can include varying environmental gas and liquid densities resulting from varying environmental temperatures and pressures. Systems that work in controlled conditions may not work when subjected to actual working conditions. Thus, there is a need for laboratory tool that can simulate arbitrary backscatter of an optical signal, and thus simulate laser operation under field conditions. Such a system would aid in the design of practical laser systems, and would thus be most welcome also.